MONITORING OF FATIGUE DEGRADATION IN AUSTENITIC STAINLESS STEELS

1 MONITORING OF FATIGUE DEGRADATION IN AUSTENITIC STAINLESS STEELS D. Kalkhof, M. Niffenegger, H.J. Leber During cyclic loading of austenitic stainless steel, it was observed that microstructural changes occurred; these affect both the mechanical and physical properties of the material. For certain steels, a strain-induced martensite phase transformation was seen. The investigations showed that, for the given material and loading conditions, the volume fraction of martensite depends on the cycle number, temperature and initial material state. It was also found that the martensite content continuously increased with the cycle number. Therefore, the volume fraction of martensite was used as an indication of fatigue usage. It was noted that the temperature dependence of the martensite formation could be described by a Boltzmann function, and that the martensite content decreased with increasing temperature. Two different heats of the austenitic stainless steel X6CrNiTi18-10 (AISI 321, DIN 1.4541) were investigated. It was found that the martensite formation rate was much higher for the cold-worked than for the solution-annealed material. All applied techniques — neutron diffraction and advanced magnetic methods — were successful in detecting the presence of martensite in the differently fatigued specimens. 1 INTRODUCTION Degradation of nuclear materials can affect the performance and operational life of nuclear power plants (NPPs) in an essential manner. In view of life extension efforts for NPPs, many investigations are in progress in order to assess the structural integrity of the different components. In many cases, this involves unexpected loads, which were not taken into account during the design of the components: e.g. temperature cycling arising from unforeseen stratification and mixing flow conditions. Existing methods for in-service inspection are mainly optimised for crack detection. However, materials subjected to cyclic loading already exhibit changes in microstructure before macroscopic crack initiation begins. This period covers a considerable part of the fatigue life. Monitoring of material degradation is a new initiative, with the intention to set-up a methodology for a microstructurally-based lifetime assessment. It includes the discovery of indications for ageing in the microstructure, and the measurement of the mechanical and physical properties influenced thereby. It is well known that at room temperature in some meta-stable austenitic stainless steels a partial phase transformation from austenite to martensite takes place under the influence of quasi-static or cyclic loading. Some amount of the austenitic face-centred cubic (fcc) lattice ( γ) changes to a body-centred cubic (bcc) lattice with a slight tetragonal shape: the so- called martensite ( α’). The aim of the present work is to investigate whether the amount of martensite α’ can be used as an indication for fatigue degradation. Angel [1] was the first to study comprehensively the isothermal formation of martensite induced by plastic tensile deformation in austenitic stainless steels as a function of stress, strain and deformation energy. The kinetics of strain-induced martensitic nucleation has been modelled by Olson and Cohen [2], who assumed that nucleation occurs preferentially at intersecting shear bands. Martensite formation under cyclic loading conditions has been investigated by Baundry [3], Bayerlein [4], Bassler [5] and others, and the influence of load amplitude and temperature on the volume fraction of martensite was qualitatively described. In spite of an improved understanding of strain- induced martensite in austentic stainless steels, knowledge relating to the influencing parameters, especially under cyclic loading, remains unsatisfactory, since in most studies the tests have been carried out in a narrow temperature range, or at only a few specified temperatures. Furthermore, there is a lack of information concerning the correlation between the mechanical response, martensite formation, and the microstructural processes. One reason for this is the difficulty in measuring the volume fraction of martensite, both on the surface and in the bulk of the fatigue samples. Here, advanced diffraction methods using neutron sources and synchrotron light sources offer new possibilities [6]. For technical applications, advanced magnetic techniques (e.g. the Giant Magneto-Resistance sensor GMR [7]) are now available. The investigations described in this paper are primarily aimed at analysing the influencing parameters of the deformation-induced martensite formation during fatigue. The experimental work contributes to the development of a lifetime monitoingr system for piping made from austenitic stainless steel. 2 MATERIAL Standard tensile and low-cycle fatigue (LCF) specimens were fabricated from heats of titanium- stabilised austenitic stainless steel X6CrNiTi18-10, corresponding to the German grade 1.4541 (equivalent to AISI 321). The materials were chosen for their relevance to NPP piping, and because of the meta-stable properties of the austenitic phase. The material, as received, was analysed by means of Inductive Coupled Plasma Emission Photometry (ICP- OES), within an uncertainty of 2% of the measured value (Table 1).

MONITORING OF FATIGUE DEGRADATION IN AUSTENITIC STAINLESS STEELS

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